A variety of physical conditions involve two tissue surfaces that, for diagnosis or treatment of the condition, need to be separated or distracted or maintained in a separated, spaced-apart condition from one another. Such separation or distraction may be to gain exposure to selected tissue structures, to apply a therapeutic pressure to selected tissues, to return or reposition tissue structures to a more normal or original anatomic position and form, to deliver a drug or growth factor, to alter, influence or deter further growth of select tissues, to carry out other diagnostic or therapeutic procedures either solely or in combination with one or more of the above procedures. Depending on the condition being treated, the tissue surfaces may be opposed or contiguous and may be bone, skin, soft tissue, or a combination thereof.
One such condition that occurs in the orthopedic field is a vertebral compression fracture. FIG. 1 illustrates a section of a healthy vertebral (spinal) column, generally designated as 100, without injury. In contrast, FIG. 2 shows a vertebral column 128 having a vertebral compression fracture 134.
The vertebral column 100 of FIG. 1 includes adjacent vertebrae 102, 102a and 102b and intervertebral disks 104, 104a, 104b and 104c separating the adjacent vertebrae. FIGS. 3 and 4 illustrate in more detail a normal vertebra and its attributes. The vertebra, generally designated as 102, includes a vertebral body 106 that is roughly cylindrically and comprised of inner cancellous bone 108 surrounded by a cortical rim 110, which is comprised of a layer of cortical compact bone. The cortical rim 110 can be weakened by osteoporosis and may be fractured due to excessive movement or loading or even due to routine physical activity or simply due to aging and bone loss. The body 106 of the vertebra is capped at the top by a superior endplate 112 and at the bottom by an inferior endplate 114 (FIG. 4), made of a cartilaginous layer. To the posterior (or rear) of the vertebral body 106 is the vertebral foramen 116, which contains the spinal cord (not shown). On either side of the vertebral foramen 116 are the pedicles 118 and 118a, which lead to the spinous process 120. Other elements of the vertebra include the transverse process 122, the superior articular process 124, and the inferior articular process 126.
The damaged vertebral column 128 of FIG. 2 has a vertebral body 130 of a vertebra 132 suffering from a compression fracture 134. The vertebral body 130 suffering from the compression fraction 134 becomes typically wedge shaped and reduces the height of both the vertebra 132 and vertebral column 128 on the anterior (or front) side. As a result, this reduction of height can affect the normal curvature of the vertebral column 128 and can lead to discomfort or pain.
Vertebral compression fractures affect a significant part of the population, and add significant cost to the health care system. As shown in FIG. 2, a vertebral compression fracture is a crushing or collapsing injury to one or more vertebrae. Vertebral fractures are generally, but not exclusively, associated with osteoporosis, metastasis, and/or trauma. Osteoporosis reduces bone density, thereby weakening bones and predisposing them to fracture. The osteoporosis-weakened vertebrae can collapse during normal activity and are also more vulnerable to injury from shock or other forces acting on the spine. In severe cases of osteoporosis, routine physical motion and actions as simple, for example, as bending forward can be enough to cause a vertebral compression fracture. Vertebral compression fractures are the most common type of osteoporotic fractures according to the National Institute of Health.
One technique used to treat vertebral compression fractures is injection of bone filler into the fractured vertebral body. This procedure is commonly referred to as percutaneous vertebroplasty. Vertebroplasty involves injecting bone filler (for example, bone cement, allograph material or autograph material) into the collapsed vertebra to stabilize and strengthen the crushed bone.
In vertebroplasty, physicians typically use one of two surgical approaches to access thoracic and lumbar vertebral bodies: transpedicular or extrapedicular. The transpedicular approach involves the placement of a needle or wire through the pedicle into the vertebral body, and the physician may choose to use either a unilateral access or bilateral transpedicular approach. The extrapedicular technique involves an entry point through the posterolateral corner of the vertebral body.
Regardless of the surgical approach, the physician generally places a small diameter guide wire or needle along the path intended for the bone filler delivery needle. The guide wire is advanced into the vertebral body under fluoroscopic guidance to the delivery point within the vertebra. The access channel into the vertebra may be enlarged to accommodate the delivery tube. In some cases, the delivery tube is placed directly into the vertebral body and forms its own opening. In other cases, an access cannula is placed over the guide wire and advanced into the vertebral body. After placement, the cannula is replaced with the delivery tube, which is passed over the guide wire or pin. In both cases, a hollow needle or similar tube is placed through the delivery tube into the vertebral body and used to deliver the bone filler into the vertebra.
In this procedure, the use of lower viscosity bone filler and higher injection pressures tend to disperse the bone filler throughout the vertebral body. However, such procedures can dramatically increase the risk of bone filler extravasation from the vertebral body. The difficulty of controlling or stopping bone filler flow into injury-sensitive areas increases as the required pressure increases. Thus, caution must be taken to prevent extravasation, with the greatest attention given to preventing posterior extravasation because it may cause spinal cord trauma. Physicians typically use repeated fluoroscopic imaging to monitor bone filler propagation and to avoid flow into areas of critical concern. If a foraminal leak results, the patient may require surgical decompression and/or suffer paralysis.
Kyphoplasty is another type of treatment for vertebral fractures. Kyphoplasty is a vertebral fracture treatment that uses one or two balloons, similar to angioplasty balloons, to attempt to reduce the fracture and, perhaps, restore some vertebral height prior to injecting the bone filler. One or two balloons are typically introduced into the vertebra via bilateral transpedicular cannula. The balloons are inflated to reduce the fracture. After the balloon(s) are deflated and removed, leaving a relatively empty cavity, bone cement is injected into the vertebra. In theory, inflation of the balloons may restore some vertebral height. However, in practice it is difficult to consistently attain meaningful and predictable height restoration. The inconsistent results may be due, in part, to the manner in which the balloon expands in a compressible media, such as the cancellous tissue within the vertebrae, and the structural orientation of the trabecular bone within the vertebra, although there may be additional factors as well.
In response to these disadvantages, the treatment of vertebral compression fractures was advanced by methods of inserting an implant into a fractured vertebral body to distract the superior and inferior endplates, thereby restoring proper height to the vertebral body. Typically, bone filler or bone cement is also injected into the vertebral body after the implant has been inserted.
A challenge in the use of a posterior procedure to install spinal prosthesis devices is that a device large enough to contact the end plates and expand the space between the endplates of the vertebral body must be inserted through a limited space. FIGS. 5-13 generally illustrate a known implant or distraction device 136 which is suitable for implantation into a fractured vertebral body. The distraction device, generally at 136 (only a distal portion of which is shown in FIGS. 5 and 6), is comprised of an elongated member 138. As illustrated in FIGS. 5 and 6, the distraction device 136 has a generally rectangular cross-sectional shape defined by a top surface 140, a bottom surface 142, and opposed first and second sidewalls 144 and 146. The elongated member 138 includes a plurality of spaced apart, laterally extending projections or teeth 148, with a recess or slot 150 defined between adjacent projections 148. The distraction device 136 is made of a rigid material and is substantially rigid or incompressible in a first dimension or direction between the top surface 140 and the bottom surface 142, and substantially flexible in a second dimension or direction that is generally perpendicular to the first dimension and extends along the length of the distraction device 136. The distraction device 136 is typically made from biocompatible materials that are suitable for long term implantation into human tissue in the treatment of degenerative tissue, trauma or metastatic conditions or where a tissue distraction device is needed.
FIGS. 7-11 illustrate one method of deploying the distraction device 136 over a guide wire 152, wherein the distraction device 136 and the guide wire 152 are deployed incrementally from a delivery cannula 154. The distraction device 136 includes a center bore or passageway that accepts the guide wire 152 for slidably mounting the distraction device 136 onto the guide wire 152. Prior to deployment (i.e., while present in the delivery cannula 154), the distraction device 136 is constrained by the shape of the cannula and guidewire and typically has a generally linear pre-deployed configuration, as illustrated in FIGS. 5 and 6.
Referring to FIG. 7, a distal portion 156 of the guide wire 152 is advanced out of the distal end portion 158 of the delivery cannula 154 and into a treatment site (i.e., a vertebral body 106, as shown in FIG. 12). At least the coil-shaped distal portion 156 of the guide wire 152 is preferably made of a shape memory material, such as a nitinol or a polymer having shape memory characteristics, so that the guide wire 152 can be deformed into a generally straight configuration within the delivery cannula 154 prior to or during deployment of the guide wire 152 into the treatment site. When the distal end portion 156 of the guide wire 152 is advanced out of the delivery cannula 154, and is no longer constrained by the cannula, the guide wire resiliently returns from its straight constrained configuration to its predisposed coiled configuration.
Next, the distraction device 136 is advanced over the curved distal portion 156 of the guide wire 152 (FIG. 8) while the guide wire 152 is held in place with respect to the delivery cannula 154. The distraction device 136 may be advanced along the coiled distal portion 156 of the guide wire 152 by pressing a pusher or plunger member in the delivery cannula 154 (not illustrated) distally. The distraction device 136 has sufficient flexibility to follow along the contour of the guide wire 152. The curvature is such that the teeth or projections 148 of the distraction device 136 remain spaced apart and do not abut, leaving a passageway therebetween for the injection of bone filler or cement following insertion.
The guide wire 152 is then further advanced out of the delivery cannula 154 (FIG. 9) while maintaining the distraction device 136 in place relative to the delivery cannula 154. This has the effect of extending the distal portion 156 of the guide wire 152 farther beyond the distal end 160 of the distraction device 136. Thereafter, the distraction device 136 is then further advanced over the guide wire 152 (FIG. 10), for example to bring the distal end 160 of the distraction device 136 into the proximity of the distal-most end of the guide wire 152. The incremental deployment of the distraction device 136 via the alternating advancement of the guide wire 152 and then the distraction device 136 continues until the distraction device 136 defines a coiled support structure 162 with the desired height (FIGS. 11 and 12). Alternatively, the guide wire 152 may be fully deployed, followed by the distraction device 136 being advanced out of the delivery cannula 154 and along the guide wire 152 until the support structure 162 attains the desired height. They may also be able to be advanced simultaneously.
Hence, when deployed into a vertebral body 106, the distraction device 136 follows the curved path defined by the guide wire 152 to define a spiral or helical structure 162 that extends substantially vertically within the vertebral body and serves to actively separate or support (or both) the opposed endplates 112 and 114 of the vertebral body 106, as shown in FIG. 12, for example, by engaging or pressing against the opposed end plates. The support structure 162 typically defines a helical configuration with a tight pitch forming an essentially hollow cylinder. Each coil, turn, or winding may have little or no spacing between adjacent windings, although some limited spacing may be employed. Because the distraction device 136 is therefore substantially rigid in the first dimension between the top and bottom surfaces 140 and 142 in this deployed configuration, the support structure 162 provides a relatively stiff support along the axis of the spine and leaves an internal volume or core of relatively undisturbed cancellous bone inside the hollow cylindrical structure, as well as largely undisturbed cancellous bone between the structure and the cortical rim.
After the support structure 162 has been formed, if desired, the guide wire 152 can be removed from the deployed distraction device 136. Alternatively, the guide wire 152 can remain in place within the distraction device 136 to further strengthen and stabilize the support structure 162. In such usage, the proximal portion of the guide wire 152 is severed from the remainder of the guide wire 152. In either situation, no void or cavity is formed in the vertebral body, and cancellous bone is not compressed or compacted, as in Kyphoplasty.
As mentioned above, the support structure 162 includes or defines an innerspace, core, or resident volume 164 (FIGS. 11 and 13). As used herein, “resident volume” refers generally to a structural characteristic of a support structure or deployed distraction device. The resident volume is a volume that is generally defined by the distraction device when it is in the deployed configuration, and as illustrated is generally a cylindrical-shaped volume extending between the end plates and located within the windings of the distraction structure.
With the distraction device 136 deployed in the vertebral body 106, a flavorable bone filler or other material may be injected into the vertebral body 106 in any of a variety of ways. For example, if the guide wire 152 has been retracted from the center bore of the distraction device 136, flowable filler material (e.g., bone cement bone augmentation material or filler, or other diagnostic and/or therapeutic material) may be injected through the center bore of the distraction device. As noted above, the teeth or projections 148 preferably do not abut in either the insertion or deployed configuration and remain spaced apart to allow flowable material to flow into the cancellous bone within the resident volume via the slots 150 of the distraction device 136. The flowable material preferably interdigitates with the cancellous bone tissue 108 located within the resident volume 164, and the tubular configuration of the support structure 162 retards extravasation of filler material out the resident volume 164 either anteriorly, posteriorly, or laterally. More detail regarding distraction systems of this type can be found in U.S. Patent Application Publication No. 2008/0234827 to Schaller et al., which is incorporated herein by reference.
While the above devices and methods represent a significant advance in treatment of vertebral compression fractures, advances have continued, particularly in terms of controlling and limiting the spread of bone filler or other flowable material injected into the vertebral body to augment the implant. For example, the slots 150 of the distraction device 136 in the deployed configuration (FIG. 13) remain open in both the radially inward and axially vertical directions. Such slots 150 allow filler material to be injected inwardly into the resident volume 164, but also allows an amount of filler material to be injected in other directions, such as upwardly and downwardly in the direction of the axis of the spine which could potentially lead to some filler material unintentionally flowing into space outside of the structure and toward the intervertebral space. Further, due to the size and configuration of the slots 150, it can be a challenge to reliably deliver filler material to the distal end of the support structure 162, which can affect the uniformity with which the filler material is injected into the resident volume 164. There may also be, as mentioned above, a potential for cement “bypass,” wherein the filler material flows around the upper and lower ends of the structure, into the surrounding cancellous bone tissue.